The present invention relates to a power converter such as an inverter or a power storage, and particularly relates to a power converter of a pulse width modulation control system with an electric current detector using a shunt resistor.
Inverters have been used widely for operating AC motors such as induction motors, and recently used also as controllers for power sources of carriages. Thus, the advantage of adjustable speed operation by using the inverters may be enjoyed sufficiently.
For the control of an inverter, detection of a load current may be required. A hall element type current sensor 28 or a shunt resistor 13 with a detection circuit 18 has been conventionally adopted for the detection of the load current.
The hall element type current sensor 28 is a current sensor in which a hall element is provided in a part of an annular magnetic substance, and an electric wire supplied with a load current is wound around or passed through the magnetic substance so that magnetic flux generated by the load current is converted into a voltage by the hall element. In this case, there is a merit that a detection signal electrically isolated from an electric circuit which is a target to be detected can be obtained.
Similarly, the shunt resistor 13 and the detection circuit 18 are defined as follows. That is, a resistor inserted in series into an electric circuit in which a load current flows is a shunt resistor. A voltage drop appearing between the opposite terminals of the shunt resistor 13 due to the load current. In order to form a detection signal from the voltage drop, a circuit is used as the detection circuit 18. The shunt resistor 13 with the detection circuit 18 can be provided with considerably low cost. Thus, such circuits have been conventionally used widely.
FIG. 11 shows an example of the background art in which both the hall element type current sensor 28 and the shunt resistor 13 with the detection circuit 18 have been applied to a power converter of a PWM (Pulse Width Modulation) control system as a target. Here, both the hall element type current sensor 28 and the shunt resistor 13 with the detection circuit 18 are illustrated for the sake of description. Actually, it will go well if either the hall element type current sensor 28 or the shunt resistor 13 with the detection circuit 18 is provided.
In FIG. 11, a main circuit is constituted by a converter (power rectification portion) 14, an inverter (power inversion portion) 15 of a PWM control system, and a smoothing capacitor 16. The converter 14 is constituted by a diode rectifier. DC power outputted from the converter 14 is supplied to the inverter 15. The capacitor 16 is connected to a DC circuit between the converter 14 and the inverter 15.
Then, when AC power is supplied to the converter 14 from a commercial power source 29 as a power source, DC power smoothed by the capacitor 16 is supplied to the inverter 15. Here, semiconductor switching devices 5 represented by IGBTs (Insulated Gate Bipolar Transistors) in the inverter 15 are PWM-controlled so that the DC power is converted into AC power with a specific voltage and a specific frequency. As a result, power with a variable voltage and a variable frequency is supplied to a load such as an induction motor.
As shown in FIG. 12, also in a power converter in which DC power outputted by a power storage 30 such as a battery is supplied to the inverter 15, semiconductor switching devices 5 of the inverter 15 are PWM-controlled in the aforementioned manner so that DC power is converted into AC power with a specific voltage and a specific frequency. As a result, power with a variable voltage and a variable frequency is supplied to a motor 17 as a load, such as a power source for a carriage, a cooling fan of a cooler, a pump driving motor for circulating cooling water, a hydraulic pump driving motor for hydraulic apparatus, or a compressor driving motor for an air conditioner.
At this time, on (conduction) and off (interruption) of the semiconductor switching devices 5 in the inverter 15 are controlled in accordance with PWM signals by a computer 19 through a driver circuit as shown in FIG. 11. To this end, the value of a current flowing in the motor 17 which is a load, that is, the value of a load current is required for the control by the computer 19.
To detect the value of the load current, there are two methods as described previously. That is, one is a method using the hall element type current sensor 28 and the other is a method using the shunt resistor 13 and the detection circuit 18.
First, when the hall element type current sensor 28 is used, this current sensor is connected in series between the inverter 15 and the motor 17 which is a load. The detection result by the current sensor 28 is A/D converted and supplied to the computer 19.
On the other hand, when the shunt resistor 13 and the detection circuit 18 are used, the shunt resistor 13 is connected in series between the capacitor 16 and the inverter 15. A voltage drop appearing due to a load current flowing in the shunt resistor 13 is A/D converted through a filter, an amplifier and so on, and supplied to the computer 19. The shunt resistor 13 may be connected in series between the inverter 15 and the motor 17.
The shunt resistor 13 is, generally, of a sheet-like resistive material 6 made of manganin material (alloy of copper and manganese) excellent in temperature characteristics. The sheet-like resistive material 6 is formed into a predetermined shape by punching or by etching after fixed attachment to an insulating layer 4. A shunt resistance 8, main electrodes 7 for making a load current flow into the shunt resistance 8, and detection electrodes 31 for detecting a voltage generated in the shunt resistance 8 are formed from the same resistive material. As shown in FIGS. 13A and 13B, the shunt resistance 8, the main electrodes 7 and the detection electrodes 31 are mounted on a heat radiating base plate 1 of a power module through the insulating layer 4. The heat radiating base plate 1 is superior in heat radiation characteristics, and the semiconductor switching device 5 of the inverter 15 is mounted on the base plate 1.
Heat generation in the shunt resistor 13 occurs in both the shunt resistance 8 and the main electrodes 7 because the load current flows into the shunt resistance 8 and the main electrodes 7. The generated heat flows into the heat radiating base plate 1 so that the temperature increase is suppressed.
The length, width and thickness of the shunt resistor 13 using the sheet-like resistive material 6 are defined as follows. That is, the length direction of the shunt resistor 13 is defined as the direction in which a detection current flows. The width direction of the shunt resistor 13 is defined as the direction perpendicular to the length direction. The thickness direction of the shunt resistor 13 is defined as the direction perpendicular to the insulating layer 4.